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How to recognize the horizontal winglet in the Horten design, or the experimental glider in this video?
Comments below the NASA Armstrong Flight Research Center video Proving Prandtl- With A Twist! incclude:

This video is entirely a hidden jewel in youtube, it deserves much more attention than what it has had until now. It is both educational and inspirational!

It's a clever, non-obvious idea, using the washout to eliminate adverse yaw...

What, and where is the Horizontal wiglet discussed in the video, or in the Horten design discussed there as well?
When I look at the various images in the video, I just see a flat wing as far as the shape is concerned; what distinguishes the winglet from the rest of the wing?
edit: I see a break in the wing near the end, but as far as I can see, the shape or orientation doesn't change from what the wing would look like anyway. What is it that makes the end of this wing a winglet?

I just watched the second video. This NASA academy brazenly claims to have newly discovered what we have known for 80 years (well, if we looked in the right places, that is). Another instance of NASA marketing overselling trivial "discoveries". And, of course, the same falsehood about tip vortices creating induced drag is repeated again.

"60% increase in aircraft efficiency" (video at 6:42) is a very,very,very,very big claim to make.

You can't from the planform alone.
First, winglets are no magical device. Calling the wing with bell-shaped lift distribution one with horizontal winglets tries to free-ride on the mystique that NASA marketing has created around the winglet. But the physics behind it are rather mundane and the thrust which is created by the outer wing is a bit of payback for the higher losses at mid-span from a steep lift gradient over span.
Next, all wings create thrust if you define it narrowly enough. This comes from the suction force on the forward upper side of the airfoil and is called leading edge thrust.
Induced drag is the backward tilt of the aerodynamic forces and is caused by lift creation. The least amount of drag for a given amount of lift and wingspan can be achieved with the elliptic distribution over span. The bell-shaped distribution creates more drag for the same lift and wingspan, since it has higher spanwise lift gradients at mid-span.

What is it that makes the end of this wing a winglet?

This is a matter of definition. The so-called winglet area is where the wingtips carry only little positive or even negative lift. Like in a winglet, this gives the local lift a forward component which works like the opposite of induced drag. Call it induced thrust, if you want: This is what is common to winglets and the negatively loaded wingtip. That in turn is caused by wing twist and local control surface deflection. You cannot see from the top view how lift is distributed over span.
But the bell-shaped lift distribution has some interesting advantages:

Since most lift is created near the wing root, the spar bending moment can be kept low for a given amount of lift. This allows for a lightweight wing structure and is especially important for large aircraft.
With aileron deflection, the lift distribution on the up-going wing becomes nearly elliptical while the one on the down-going wing becomes even worse, increasing induced drag there significantly. This reduces adverse yaw such that no vertical tail is needed.

Sounds great, doesn't it?
Actually no, it doesn't when you take a closer look:

Due to the low maximum lift coefficient of flying wings, the wing surface of a flying wing needs to be much higher than that of a conventional configuration of the same landing speed where a tail surface allows the use of powerful trailing edge flaps, raising wing weight and drag substantially.
The bell-shaped lift distribution is like flying all the time with spoilers half deployed. Aileron deflection retracts the spoiler on the up-going wing and extends it fully on the down-going wing. Kind of like the split ailerons of the B-2. I think it is better to only use spoilers during manoeuvring. Also, the Horten flying wings were all known for marginal directional stability, especially at high speed when sweep did not help much. It was too little to even compensate for unsymmetric thrust. A fin or added artificial stabilization would be highly advisable.

This is more than I bargained for, but I will hunker down and try to understand it all now. Thank you for taking the time to include so much into one post!

So what is your view on that they just lengthened the span and called this a "horizontal winglet"?

@jjack: No, also the right twist is needed, correctly called washout. That's why they make the pun on "Prandtl with a twist".

And you say that winglets generate thrust? What about the drag that they also generate? Do we have a net thrust?

@PeterKämpf I get that "Prandtl with a twist".

@jjack: Winglet net thrust is indeed possible at low speed, when friction is low and induced drag is high. YMMV - for most cases they add more drag. They help most for prolonged flight at high lift coefficient and in case of span restrictions.

I have several posts and snarky comments on here that describe the function of winglets precisely as outlined in this video; that is, they exploit the circulation around the tip to generate thrust (like sails on a boat, which is why they were originally called "tip sails").  Almost all descriptions talk vaguely about how they give a reduction in induced drag. This is the first time in a long time I've seen it explained so clearly, and is great to see.
Anyway, remembering that a winglet is a flying surface that generates thrust from tip circulation, what they have done here is simply a flat tip extension with its incidence set (more nose down than a normal wing tip) to exploit the same circulation, kind of earlier in the circular movement of the flow (at 9 o'clock instead of 12 you might say).  This placement seems to generate a much stronger thrust component from the vortice than a vertical winglet, so strong that it is enough to completely cancel out the increased drag from the nearby down aileron.
This means that the elimination of adverse yaw this way, along with the sweep back that provides a natural weathervaning tendency, allows you to completely do away with rudders.
Coping with asymmetric engine thrust is another job of rudders not addressed here and a multi-engine aircraft would still need some kind of asymmetric thrust compensation device, but besides that, it seems brilliant.

Thanks for your answer! I'm not very familliar with winglets; all I see is a wing in the image. What makes the winglet different from just the end of the wing? What delineates the end of the wing and the beginning of the winglet? Would it be articulated forward or backward when in flight?

Is it possible then to add a short explanation to "how can it be recognized" in the context of this test glider? I just see a wing with a gap in it, but I don't understand what makes the end into a winglet, and not just the end of the wing.

Maybe they have tried winglets as part of their design iteration and then decided to leave them off, or rather bend them downward thus obtaining a bigger wingspan.
"Horizontal winglet" seems to be a term used in windtunnel studies to indicate "the winglets are bent down" as opposed to 60 degree, etc. winglets.
The difficulty with these "horizontal winglets" is that essentially you have a different wing from the one you started out with, one with a longer span. This is sort of like cheating on yourself.
So, there are no winglets in the given design. They just increased the wingspan and call this "horizontal winglets", which is a misnomer.
And I also think their explanation with regards to thrust being created by the winglets is wrong.

I think you appreciate my confusion better than most, thank you! If you get a chance and can find a link or an image that helps me to visualize what you are explaining, that will be greatly appreciated. Thanks!

I've enjoyed reading all of these posts.
I assume many have read this paper, but if not, one may enjoy and find it helpful here.
A friend shared this 2016 NASA Technical Report with me, and I later ended up on this thread.

On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds. NASA/TP—2016–219072

For nearly a century Ludwig Prandtl's lifting-line theory remains a standard tool for understanding and analyzing aircraft wings. The tool, said Prandtl, initially points to the elliptical spanload as the most efficient wing choice, and it, too, has become the standard in aviation. Having no other model, avian researchers have used the elliptical spanload virtually since its introduction. Yet over the last half-century, research in bird flight has generated increasing data incongruous with the elliptical spanload. In 1933 Prandtl published a little-known paper presenting a superior spanload: any other solution produces greater drag. We argue that this second spanload is the correct model for bird flight data. Based on research we present a unifying theory for superior efficiency and coordinated control in a single solution. Specifically, Prandtl's second spanload offers the only solution to three aspects of bird flight: how birds are able to turn and maneuver without a vertical tail; why birds fly in formation with their wingtips overlapped; and why narrow wingtips do not result in wingtip stall. We performed research using two experimental aircraft designed in accordance with the fundamentals of Prandtl's second paper, but applying recent developments, to validate the various potentials of the new spanload, to wit: as an alternative for avian researchers, to demonstrate the concept of proverse yaw, and to offer a new method of aircraft control and efficiency.

Welcome to Stack Exchange! I've added the abstract so that this isn't a link-only answer, and linked to the main NTRS page for the paper rather than directly to the PDF. Some people browse on limited mobile plans and don't want to end up unexpectedly downloading PDFs. Feel free to edit further or add more information. Thanks!

Good contribution, but even Prandtl himself acknowledged in his 1933 paper that most then current designs would already employ his more advanced bell distribution. The NASA paper makes it look as if nobody had noticed when in fact most designs for the last 90 years already implemented it. It is sickening to see how NASA marketing is distorting the facts and creates an alternate history just to elevate their own mediocre results.

Great work from the NASA team and an interesting way of thinking in 3 dimensions.
A look at bird wing anatomy shows how they decrease lift and increase drag on the same side:  by using their "wrist" to pivot their wing tip leading edge down.  You can do this by sticking your arm out and rolling your wrist.  Conventional aircraft need rudders to counteract "adverse yaw" created by downward pointing aileron (higher AOA, higher lift) on opposite side of turn. This is the "coordinated" turn.  A spoiler on the same side is more "bird like" and can be found on the venerable B52.
But before we go throwing away our vertical stabilizers and rudders it is very important to study cross wind performance.  A dihedral aircraft with roll away from the wind and get blown sideways from a strong gust. A "weathervane" motion of the tail speeds up the leeward wing, helping mitigate the roll.  Birds mitigate cross wind roll by anhedralling their wing tips (again the wrist).
This relationship of aircraft dihedral and vertical stabilizers is expressed in discussions of "Dutch roll" (vertical stabilizer too small) and spiral instability (too large). This may be why the B52 reduced, but did not eliminate theirs.

Looking at the picture of the glider, the orientation of the hinges indicates that the winglets should provide directional control, replacing the rudder as well as the ailerons. The original Horten aircraft had spoilers to perform the same role as the rudder in a conventional design.

Thanks, but I still can not visualize the difference between "flat winglets" as discussed in the video say between04:30 and 05:45, and a wing of the same length without "flat winglets".


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Do pilots on approach receive automated warnings of sudden windshear or seismic motion?
I've read about Windshear Awareness and there is mention of a Predictive Windshear System (PWS) which I believe is a function of the radar system of the aircraft itself.

To complement the reactive windshear system and provide an early warning of potential windshear activity, some weather radars feature the capability to detect windshear areas ahead of the aircraft.
This equipment is referred to as a Predictive Windshear System (PWS).

Are there any gound-based systems that detect possible windshear situations that generate automated warnings directly to planes on approach? Or is the information only distributed verbally by controlers  (e.g. LLWAS)?
Albeit exceedingly rare, are there any automated warning systems to pilots indicating other sudden dangers, for example large seismic motion? For example, the automated stopping of high-speed trains - sometimes before the sesimic wave even reaches a particular train - is being explored (in places where earthquakes and high speed trains are both frequent).

There is at least a case for Hong Kong. The detection system feeds automatically the ATIS and the D-ATIS. D-ATIS is a digital ATIS, the information are sent to the aircraft using ACARS. See HKO booklet. There are research ongoing, and data transfert is being extended, the near future will certainly be more automated.

@mins thanks for that! It's a good place to start.

Windshear, yes. Seismic events, no.
Most major U.S. airports have a LLWAS -- Low-level Windshear Alert System, which is essentially an array of anemometers around the airport which detect wind speed and direction.  The values from each station are processed by a computer that uses algorithms to detect a windshear or microburst event, and it notifies controllers in the tower.  As a simple example, if the anemometers around the perimeter of the airport are all pointing toward the center of the airport, that's a strong indication that there is a microburst over the center of the airport, with air descending into the ground and then spreading out in all directions.  Obviously, the algorithms can detect cases beyond just that.
And, many airports supplement that system with Doppler radar as well.
Regardless of how they're generated (LLWAS with anemometers, Doppler, pilot reports, visual observations of blowing dust, etc), the warnings are broadcast by the tower controllers, rather than by any automatic system.  In the aircraft, aural warnings from the GPWS (reactive windshear -- noting changes in groundspeed and sink) and the EGPWS (predictive windshear -- returns from the aircraft weather radar) are automatic, but they don't block a radio frequency.  Automated alerts over the tower frequency might block an even more important transmission, so it's left to the controller's judgement when to broadcast the alerts.  (They're usually pretty prompt!)
No airport I'm aware of has a system similar to what the OP described for seismic events.  I would assume that the risks to a high-speed train from an imminent earthquake -- i.e. the train may derail at high speed with catastrophic results -- are far greater than the risks to aircraft.  I've never read anything about what happens when you're landing or taking off in an earthquake, but I'd doubt that anything other than the most severe would have much effect.  And if the earth is shifting enough that you can't keep the aircraft on the runway, well, it's a really bad day for pretty much everybody in that state that day!

OK this is a helpful answer. I hadn't thought about frequency-blocking but that makes sense. Also, considering that trains spend all of their at-speed time (hopefully) on the ground while moving aircraft almost always not on the ground, sounds like a low-priority item.

About sismic events, one problem is that they can easily liquefy the soil. It's not a matter of strength, but of seismic wave frequency. This phenomena is used in vibro-sinking of steel profiles. See soil liquefaction. A magnitude 5 or 6 earthquake is sufficient and not uncommon.

@mins When this happens, I think you've got much bigger trouble than one landing aircraft :-)

At some U.S. airports, there is a system called TWIP (Terminal Weather Information for Pilots) where pilots can access weather information at the terminal via ACARS, including windshear or microburst alerts ongoing at the airport.

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What would the density profile of this shock wave look like, roughly?
The YouTube video titled Shock Wave Formation in Transonic Flight shows a hand-held video of a visible shock wave extending from the top of a jet engine near the front. The shock wave is visible by its refraction of features on the ground (muddy river bed) cause by the large changes in air density over short distances.
I would like to simulate the optics of this strictly for fun and general interest. If one made a simple plot of the density as a function of position crossing the shock wave, roughly what shape would have?
I've plotted six simple examples of shapes, perhaps one is appropriate, but perhaps something else is better.
below: GIF made from this video around 00:31.

below: some example guesses of the density behavior across the part of the shock wave seen to refract ground features in the video.

below: Description from this video.

Published on Oct 18, 2012
Formation of a shock wave around the engine cowl of a Boeing 767 at high speed.
The aircraft itself isn't going supersonic, but it's going fast enough (probably around Mach 0.8) that the local airflow around the engine cowl is supersonic, in a small area. This is called "transonic," and results in the formation of a shock wave at a right angle to the surface of the plane. The air in front of the shock is supersonic, and the air behind it is subsonic.
The shock wave is visible because of the abrupt change in the density of the air. The change in density causes a change in the refractive index of the air, and so the scenery behind it is optically distorted. This is similar to how a spoon in a glass of water looks "bent" at the water's surface, due to the difference between the refractive indices of water and air. It helps that this is seen nearly edge-on... if I had been a couple of rows further forward or back, it might not have been visible.

Not an answer, but this document shows an abrupt transition.

@mins very nice! So pretty much something like either the first or the third curve in my example guesses. As the video caption explains, it's like a simple transition from one index of refraction to another. I've just got to generate it in 3D (make a cone around the engine axis) and see what happens. I can always add a transition thickness of a millimeter or centimeter and see if that changes the appearance.Thanks!

Another pressure distribution graph (middle) at the end of this article.

The shock observed is effectively the same as the one over a wing. Compression waves are normally considered of infinitesimally small thickness, although due to the non-continuous nature of gases made of particles, this normally makes the shock on the same order of magnitude as the mean free path of a gas molecule (See here for the values at different altitudes).
For visualization's sake, here's a CFD from a transonic NACA0012 airfoil with a shock on the suction side, and a plot of the density gradient on its surfaces. The step in density is not a pure discontinuity, as it would be physically, due to the relatively coarse mesh used, but I hope it still delivers the idea.

Excellent! Thanks for finding my question and posting a very helpful answer. What is the scale for density? Is it normalized to the density at infinity? I'm trying to understand why it's >1.5 at the left and right edges.

The freestream density was set to 1.225kg/m3, which is approx. sea-level air density. The leading edge is under compression due to the dynamic pressure but with regards to the trailing edge, my guess is that skin friction has converted some of the dynamic pressure into static pressure.

I see, it got hot. OK thanks!

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Where to find the parking coordinates of the aircraft in the Apron?
Is it possible to find out the coordinates of where the aircraft park in an the Apron? I mean some sort of list for each terminal or aeronautical chart.
I was thinking about the AIP (Aeronautical Information Publication) but i could not find it.

Which airport in particular?

A point to note is that the AIP airport chart as well as Jeppessen and other charts have a Lat/Long grid marked in Degrees and Minutes with notches every 10th of a Minute. Jepps actually have only notches marked on the edge of the diagram. It's good to also know that 1 minute of Latitude = 1 Nautical Mile (But not Longitude). FMC's need an accuracy of tenths of a degree.

The airport charts usually contain the coordinates of all parking positions. The charts that are available from the FAA AIP (e.g. here for JFK) only contain a single ground chart with the airport diagram:

These do not include the coordinates. But my Jeppesen charts contain coordinates for all parking positions. E.g. some gates at JFK:

They can either be found in dedicated charts for airports with many gates (like JFK shown above), or they are included on the airport overview chart for smaller airports with fewer gates.
Aircraft navigation databases also typically include the positions, so the Inertial Navigation can be initialized with a position based on airport code and parking position.
As far as I know, there is no free resource that would list the coordinates though.

Perfect exactly what i was looking for! Thanks a lot

... or this one for EGLC (London City) from NATS UK. For Germany, for instance, such information is not available for free.

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How to find out how many aircraft depart/land from a country per day?
I am trying to get the following information. I would like to know for each day how many aircraft have taken off and departed from a specific country. This information should be up to date and possibly have also some historical time series as well (for example the last month).
I know i could get such information by paying some services like Flight Aware or FlightRadar24 but I need to do it for free.
I tried OpenSky but the API doesn't offer much information (just time, lat/lon, origin).
Do you know a possible way to gather such information? It doesn't need to be machine readable as long it has the info.
Thanks in advance.

The OpenSky API features arrivals by airport and departures by airport for a specified time window. Could you specify what else you need?

Lat/Lon (if that were all that was available) should be more than enough to localize to a country. Any GIS system would require than and something as simple as PowerBI or Tableau will plot the data on a map for you with just that info.

@FreeMan It's not even necessary to deal with coordinates: estDepartureAirport contains the "ICAO code of the estimated departure airport". The first two letters of the ICAO code tell you pretty much, which country you are in. Some countries have more than one prefix, but I'm sure, you'd find a list somewhere on the net.

@svoop I read, recognized, and understood your initial comment, therefore I added "(if that were all that was available)". Just providing another option for the OP.

You need to define what size/usage aircraft you’re interested in - scheduled airlines will be easier to find out than overnight cargo or general aviation.

@svoop Nothing more actually... I would like to know how many aircraft departed from china and arrived to china on a daily basis. Mostly interested in civil aviation


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What happend to the airframe of the Aloha Airlines Flight 243?
What happened to the airframe of Aloha Airlines Flight 243, after the explosive decompression?
I know that the structures were studied for a while after the aircraft safely landed in  Kahului Airport, but what happened after that?

It was written off and dismantled.

@Simon Thanks for your help!

You're welcome, and welcome to Stack Exchange Aviation.

Thank you. That was my first question, I had always wondered what happened to the airframe, and now I know!

Accident report of Aloha 243

According to Planespotters, it appears that the airframe in question was destroyed in August 1988.

The link is broken.


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Is there a backup to fuel inerting systems?
I've heard it said many times that "everything has a backup" in commercial planes.
What about the fuel inerting system (injecting nitrogen in the fuel tank). Does it have a backup? Is it somehow failsafe all by itself?

Well the inerting system is itself a backup of keeping ignition sources out of the fuel tanks.

I think "everything has a backup" is an overly broad generation. It's safe to say that the redundancy of all systems are carefully considered as part of an aircraft's design, but that doesn't mean everything literally has a backup (there is no backup for, say, the wings). The incredibly rare rate of fuel tank explosions can itself be the backup for the fuel inerting system.

read up on the Swiss Cheese Model to understand how aircraft safety works  https://en.wikipedia.org/wiki/Swiss_cheese_model

Short Answer
No, it does not have a back-up, there are some fail-safe components in the system(see below), but in general the system itself is not duplicated. The system, together with a design that avoid ignition sources within the tanks, is just a part that contributes to prevent fuel tanks to ignite. An Inerting System for commercial planes have been considered since 1996 TWA accident and first regulation arrived in may 2006.
Longe Answer
The certification CS25.981 Fuel tank ignition prevention states:

(a) No ignition source may be present at each point
  in the fuel tank or fuel tank system where
  catastrophic failure could occur due to ignition of
  fuel or vapors.
(c) Design precautions must be taken to achieve
  conditions within the fuel tanks which reduce the
  likelihood of flammable vapors.

So the function that should be achieved is prevent tank ignition and there are two fronts two work on, avoid ignition source, and reduce fuel vapors (similar to the two slices of cheese that rbp was mentioning in the comments).
So the fuel ignition system is a system designed to avoid fuel vapors. As can be seen in the image below, within the systems there are some redundancies (e.g.: Dual Flow Shut Off Valve), but the system itself is not duplicated.
Moreover the system is not on the Minimum Equipment List (MEL), so the aircraft can still be dispatched with the system inoperative for 10 days.


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Does the USA government operate any business-jets with mid-air refueling capability?
Today I saw a business jet, 10 or 12 passenger or so fly over, evidently on approach to Whitehouse Naval Outlying Field (NOLF) (KNEN), said jet had some sort of probe on the nose and was painted white.  I was not able to discern any markings.  Does the USA government operate any business jets with mid-air refueling capability?  Another obvious question is: why would a passenger jet fly into NOLF?

Its possible you are looking at a test platform aircraft with an instrumentation "spike" on the nose to take clean air measurements ahead of the aircraft. Most mid-air refueling systems are not spikes but nozzle receptacles, the only time I really see probes used is on helicopter's.

Welcome to aviation.SE! It may have been a pitot tube, we have a few questions about that: here, here, here.

@RonBeyer -- outside the US, probe and drogue dominates (USAF is really the only bunch that prefers the boom)

The answer is Yes civilian aircraft register the one in the picture above is the learjet25 operated by [Calspan][1] is an inflight simulator, This aeroplane used to travel worldwide to charter the aircraft for various military and Civilian Test Pilot School like NTPS ITPS (check wikipedia)
Last evaluation I did was simulating PIO (Pilot Induced Oscilation)  in simulating the behaviour of one of unstable fighter aircraft that crashed during prototype.
the Idea of inflight simulation is to check the safety features, with as close as its fidelity as possible to check the stability and control characteristics.
Imagine that this small aircraft can operate with mathematical model of any aeroplane even A380 if the mathematical model is available.
In this case, the refuelling probe install, is to evaluate HQR Handling quality Rating) of the certain aircraft in performing air to air refuelling to avoid accident / incident. I have investigated one of the accident of military aircraft that was not equipped with FDR and one of the technique is to use rare light bulb analysis.
Nevertheless, this aircraft is very popular flight testing device in either test pilot school or even aircraft research and development of aircraft manufacure, we have tested prototype of N250 using this calspan Lear25 and it was a great tools


text:
Is the TIS-B traffic filter on board or on the ground?
This question is asked in the context of small certified GA aircraft in the US.
The TIS-B traffic that is displayed on board is a subset of all the TIS-B traffic in the area. It's often described as a hockey puck surrounding your aircraft where other traffic that is outside the puck is not displayed.
My question is whether the algorithm that filters what does or doesn't get displayed resides on the onboard box or the ground station.

Does the ground station (which knows your position because of your ADS-B out) send a custom set of traffic suited for your hockey puck? OR...
Does the ground station send out all traffic it is aware of and your on-board box filter only the ones in your hockey puck and send them onto your display? OR...
Something else?

Since B stands for broadcast, I surmise the on-board receiver does the filtering. It makes sense to divide the work up that way, apart from anything else it makes more efficient use of radio bandwidth in the highly congested airspace where TIS-B is most useful.

TIS-B is part of the ADS-B system that will be required in class B and C airspace come 2020. The processing of traffic relevant to your aircraft is done on the ground to keep the airwaves "clean" as it would require transmission of much more data to show all aircraft and then have your local device filter it. In order for you to take advantage of your own "hockey puck", you first need to have installed in your aircraft the appropriate avionics, namely a GPS with WAAS and an ADS-B out or Mode S transponder. Your transponder will broadcast your location and altitude, then the ground station will process which traffic is relevant to you without the 3D boundaries of the "hockey puck" surrounding your aircraft, then it will broadcast that response out to the air. The key is that not only your aircraft can pick up the response - any aircraft with an operable ADS-B In system can read it and thus display traffic/give traffic alerts even if you don't have an ADS-B out. The key though is that without an ADS-B out the traffic information is not customized for you so critical information/traffic may be missing (what is transmitted is might not be in YOUR hockey puck).
Note: It is also possible for you to pick up signals directly from other ADS-B out equipped aircraft in your proximity without an ADS-B ground station to relay that information and they will display directly on your ADS-B in equipped system.

TIS-B transmissions are broadcast (the -B) which mean they are not addressed to individual aircraft. The TIS-B transmitter will not select traffic of interest for your specific aircraft, as it provides the data simultaneously to many TIS-B receiving aircraft.
If the display system in the aircraft only shows a 'hockey puck' of TIS-B traffic, then some filtering must be done in the ADS-B / TIS-B receiver on board of the aircraft.
The information contained in TIS-B transmissions covers a wide area. That is usually the area that the transmissions can be received in and then some buffer. To achieve this, some geographical filtering is done in the TIS-B server which provides data to the TIS-B transmitters. This is needed to protect the radio spectrum from unnecessary transmissions; it is useless to report the position of an aircraft that is a couple of states away. To make efficient use of the radio frequency, TIS-B transmitters may use multiple directional antennas,each broadcasting only the traffic that is relevant to its covered airspace.
Summarizing:  there is filtering done on both the transmitter side and the receiver side, but the selection of which traffic is relevant to your aircraft is done on board.

If there are a number of aircraft in one of the directional antenna's covered airspace but none of them have ADS-B out, is there going to be any TIS-B broadcasting going on in that coverage area?

Yes the information is provided regardless whether there are ADS-B transmissions from that area. Otherwise a malfunctioning ADS-B transmitter could result in loss of TIS-B traffic as well. It is possible to have a TIS-B receiver without transmitting ADS-B.

@DeltaLima that's only partially right because ADS-B data packets only supply information for a limited number of ADS-B targets... if the number of aircraft is too high it can't transmit info about all of them in a single transmission.

@Pugz ADS-B data packets supply information for at most a single ADS-B target. If the number of aircraft is greater than one, more packets are needed, i.e. more transmissions are needed. That holds for both 1090ES and 978UAT...

@DeltaLima That's exactly right, but when there are "many" targets (I forget what the number is) the ground station won't necessarily broadcast all of them to prevent congestion, hence the "customized" traffic selection transmitted by the ground station for aircraft with mode S / ADS-B out.

@Pugz you are now talking about graceful degradation during system overload (traffic levels exceeding system design capacity). That is far from normal operations and I am not sure how that was finally implemented. But IIRC not like you describe.


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What is the purpose of the extra horizontal and vertical surfaces on the tail of the Beechcraft 1900?
I am building a very large(11ft span) twin engine RC airplane that is loosely based on the Beechcraft 1900D design.
I can't help but notice the extra horizontal surfaces sticking out of the aft fuselage and the vertical surfaces hanging down from the ends of the horizontal stabilizer.
Could it be that after test flying they decided that the center of pressure needed to be moved further aft, but maybe it cost less to add new small surfaces instead of altering the design of the current surfaces?

Yes that is what is the most cost effective solution after flight testing has demonstrated that the vertical tail surface has to be increased for instance. The little winglets at the end of the horizontal tail do increase aerodynamic efficiency of the stab - plus they just look cool.

The BAe 125 stuck a bit of fin on top of the original fin design, the Beech 1900 added the extra vertical tail surface underneath the aft fuselage cone, in the form of 2 v-shape surfaces which also help the total horizontal tail area.

The best guess that an outsider can make is that the designers discovered the aircraft needed more lateral stability.
This is also apparent by the fact that the 1900D has larger ventral fins (the 2 vertical surfaces that extend below the aft fuselage) than the earlier models since it has a taller fuselage (and therefore taller cabin), in order to allow passenger walk in the aisle without crouching.
Beechcraft 1900C

Beechcraft 1900D, note the taller fuselage and larger ventral fins

Both images from https://en.wikipedia.org/wiki/Beechcraft_1900


text:
Can an experienced airplane spotter identify planes only by sound?
I come from a background in railroad watching, and often times one can identify the maker of a railroad locomotive purely by sound.  Maybe not the exact model of locomotive, but at least what type of diesel engine is in the locomotive.
Although there is a similar question here: What could cause GE and Rolls-Royce turbofan engines to sound different?, my question is whether an experienced airplane spotter could be near the end of a runway at a busy commercial airport and be successful in identifying the model of commercial jet aircraft landing and taking off only by the sound the spotter is hearing from the aircraft as it flies overhead?
To me as an inexperienced airplane spotter, they all sound similar.  But to airplane spotters, all railroad locomotives may sound the same too.  :-)

Much like your train example they may be able to identify the engines better than the airframe. A lot of engines are used across multiple airframes.

It may be possible with some incredible hearing and memory, but it is doubtful, especially since aircraft of a single type can have multiple different engine configurations/manufacturers.

Well, it is possible to deduce the general build of an airplane by the sound intensity as the quad-jet airplanes are louder than the other kinds. The B787 and B737MAX airplanes are on the quieter side due to engine design.

There used to be a DJ at one of the radio stations here would have people call in and he could identify their car from the sound of the horn. He was surprisingly good at it

I can tell the types of engines apart, but not the types of aircraft. Geared ultralight engines or radial engines are clearly different from regular piston engines. But to tell a Piper from a Cessna is not possible for me.

My parents house where I growth was just few hundred yeards from an airport late at night I can hear them even on idle. I can tell apart a few engines and since there only a small number of combinations taking off from that particular airport. I can make a educated guess on the airframe. At the time a 727 and a 737 can be easy tell apart. There are a few EMB-312 flying routinely and I can tell when they are in formation and how much P&W are running

The BBC tried this a few years ago, with mixed results: https://www.youtube.com/watch?v=T1TmIxug5Eo (I can only find the BBC Thai version online, but it is mostly in English with some subtitles.)

Yes it is possible, although it is kind of hard to prove it over the internet.1
When a teenager I lived under the approach path of Westover AFB. My friends and I had no difficulty at all distinguishing among these types by sound alone:

Douglas DC-3
Douglas DC-7
Boeing 707
Douglas DC-8
Lockheed L-188 Electra
Lockheed L-1049 Super Constellation
Convair B-36
Boeing B-47 Stratojet
Boeing B-52 Stratofortress
Convair F-102 Delta Dagger (singly or flights of two or four)
Lockheed F-104 Starfighter (singly or flights of two or four)
North American F-100 Super Sabre (singly or flights of two or four)

We only looked up when some other type flew over.
1. This line is an homage to kevin who said it first.

I also grew up next to an Air Force base and can still remember the sound of a C5 taking off. Nothing else like it in the world.

Grew up under the traffic pattern of NAS Oceana in Virginia Beach. Could identify an F4, A6, A7, F14, A4, and most any other Navy plane in my sleep just from the sound.

Yes it is possible, although it is kind of hard to prove it over the internet.
Generally the type of engine (propeller / turbo-fan / turbo-prop etc.) is quite easy to distinguish as they make very different sound. I can also tell a Rolls Royce engine from others because Rolls Royce engines make a quite "harmonic" and "warm" sound as it powers up.
Now here is the fun part: I correlate the sound with the type of the plane by comparing the Doppler effect as the plane passes by. It tells you how fast the angular velocity is in relation between you and the plane. The variation in loudness gives you the approximate distance to the plane - if the plane is far away, its loudness would change slowly. Combine that with the geographic location of yourself and the flight paths of planes, it gives you the speed of the plane.
The speed of the plane is important because it tells you the size of the plane. If I hear a loud multi-prop engine moving slowly, I'd bet a transport plane, something like a C130. If it is moving quickly, I'd bet an aerobatic aircraft.
The location where I live is directly under one of the SIDs of a major airport. Planes are usually ~3,000 feet when they pass above me. One thing I notice is that long route planes like Boeing 777 or Airbus A340 makes a noticeably louder noise. This is because these planes carry a lot of fuel on takeoff, and their climb performance is restrained compared to short haul airplanes, meaning they pass above me at a lower altitude, hence the louder noise. I cannot distinguish from sound if it is a B777, B747, A330 or A340, but I know it is not a B737 or A320 unless the pilots fly slow and low.
I confess I have an unusually good hearing ability. I often hear faint sound or noises which others cannot notice. I am also a musician.

Of course. I live right at the beginning of the runway 30ILS intercept area of KOAK. I hear a lot of planes, and you can usually identify an a320 variant (like a321) by its higher pitched noise compared to the 737. The 737 sounds like a hollow whooshing sound(like one you would make with your mouth). The MD-11 and DC-10 sound like the 737 but louder. 777 has its distinct GE90 sounds. Not very many 747/787's come by though. There aren't enough 757/767 for me to actually remember and recognize their sound. Keep in mind, these planes are all probably on idle as they are getting ready for final.
Also a short story: Once I was just sitting by my computer and I heard what sounded like a huge jet outside. It sounded so loud, definitely worth of 4 engines. I didn't recognize the sound so I opened FR24, and guess what? it was a British Airways A380.
So basically after a while of matching the sound with the plane you get used to it and you just 'know' what sound they make. Just like someone calls your name and you think "hey, that guys talking to me".

A few years ago I was camping at Download Festival with a friend who is a 747 pilot. When we weren't watching bands we spent a fair bit of time in our tent playing the "identify the engine" game as aircraft took off from East Midlands airport right over our campsite. She was pretty successful, pointing out the differences between jet engines, discussing the difference between a Graunch and a Whine, such that I can now easily distinguish between a Rolls Royce and a GE engine, between 2 and 4 engines, likely airframe (based on launch noise and volume) etc. So generally, yes, you can distinguish between many types of aircraft by noise profile.
One stumped her, though - and I identified it as a C-130 (from my years in the Falklands this was a familiar sound) so we watched it go by and then got back in the tent.
The next noise we heard was something neither of us could identify. So we climbed out of the tent - as did everyone else around us. The loud, earth-shattering roar that increased in volume was the Antonov AN-225 Mriya taking off almost directly overhead. Never going to forget that one.

While some aircraft, with rare engine/prop combinations can be identified, many others cannot as the same basic engine and prop get used on many fairly similar airplanes. The prop diameter and pitch can change the sound, but similar aircraft will almost always have similar props with similar pitches. Prop RPMs are often within a similar narrow range - usually designed so that the tips are just barely subsonic at high speed, which when calculated at a specific diameter set the speed - although some have gearing, so the engine can turn faster, affecting the sound.
A Piper and a similarly powered Cessna will be indistinguishable for instance, but will have a distinct sound much different than a turboprop or a big radial, even without any doppler effect. Multiple engine aircraft also sound different than single engine aircraft, and radial engines sound different than inlines, or horizontally opposed engines or the rotaries mostly used before and during WW1.
Part of this is the small number of companies making aircraft engines - less than a couple of thousand world wide over the history of aviation, and only a handful with any large production runs over an extended period of time so a lot of airplanes used many of the same engines - and conversely, most of them were built using nearly every possible engine available, so two examples of essentially the same aircraft can sound different due to the engine - while another companies airplane may also offer the same engines, and so the ones using the same engines will sound similar.
Take a look at the listings for Waco biplanes - doesn't matter which type, but some of them could be bought with as many as a dozen different types of engines. Even now, most airliners are offered with several different engines, to match whatever a fleet is using, as they try to stick to as few different engines as possible.
In the US the most common piston engines are from Continental, Lycoming, Wright, Pratt and Whitney and either Jacobs or Kinner. In the UK, add de Havilland, Rolls-Royce, Bristol and Armstrong-Whitworth - the DH and AW both being widely exported and produced abroad, as were the Wright and Pratt and Whitney engines, and many of these have a distinctive note.
TL:DR version - sound can sometimes be used to determine the broad aircraft type but not the company or model unless it is a very rare combo, such as a B-36.

Yes and no. The heard sound depends on aircraft's speed, altitude, surface positions (flaps, AoA), gear, weather (especially wind direction), other external noise (e.g. nearby busy roads) and of course engines (big proportion).
So a "resident planespotter" like @kevin can clearly identify aircrafts very well just by sound. At a different airport with different environment conditions (especially different approach procedures), it is much more difficult.
By the way, the aircraft dependent noise (engines, gear, flaps) is studied and optimized during design of an airplane.


text:
What is an out-of-trim condition and how is it detected?
I am reviewing autopilot requirements which mention that whenever an aircraft's altitude changes as a result of air disturbance or an out of trim condition, the inertial sensors will detect this change and an error signal is developed and fed to the relevant control channel, amplified, and applied to the servo actuator so that flight control surface can  be moved.
My question is what is out-of-trim condition? Is it something related to setting trim for constant speed?

Do you know what "trim" means in terms of aircraft?

I am assuming the requirement is talking about pitch/roll or yaw trim. I am not clear on what it means when it refers 'out of trim'. My understanding of trim is that pilot uses trim tabs to relieve pressure on the cockpit flight controls. pilot can choose to trim the elevator (or stabilator) such that nose up attitude is maintained without pulling back on the yoke

Yes, basically that is correct. Put it another way - when an aircraft is "in trim" it means that the aircraft tends to fly straight when there is no pilot (it means the same thing you're saying but in layman's terms). So out of trim is obvious from this understanding: is simply means that the plane cannot maintain straight flight without the pilot.

The trim system on an aircraft relieves the control pressures so that the aircraft maintains an attitude without pressure on the control system.
When hand-flying this is very noticeable in that you constantly have to be pushing, pulling, or turning (in the event you have aileron trim) to maintain an attitude. Notice how I don't say "straight and level", you can trim an aircraft to maintain a climb/descent or other attitude as long as the trim tabs have enough authority.
Autopilots detect an out-of-trim condition the same way you do. They sense that there is pressure on the controls to maintain the desired attitude, and if this is over some threshold it will display a warning light for an out-of-trim condition. Some autopilots have a trim-up or trim-down light to help the pilot determine which way to trim the aircraft.
Here is a diagram from a S-Tec System 50 Autopilot found in many GA aircraft:

The "out of trim" condition lights are two yellow indicators on the right side of the auto-pilot instrument.
Why is this important? Mostly for the longevity of the equipment. The autopilot works by either physically moving the cables or actuating the hydraulic system. It uses power to run motors which cause the motors to heat up and use electrical power. By trimming the aircraft, you can reduce these forces required by the autopilot so that it isn't working so hard to control the aircraft.
Another good reason is that you don't want the aircraft to do something unexpected when you hit the autopilot "off" or "disconnect" switches. This is especially dangerous in IMC when you don't have a visual reference to anything. There are also conditions which can cause the autopilot to disconnect automatically, and it takes a few precious seconds to figure out what is going on before the pilot gets positive control.

So, when engine settings change which  reduces airspeed then autopilot pitches aircraft down to maintain constant speed. , correct? How does autopilot know that aircraft is trimmed for constant speed. Is it because of settings in flight control panel?

Depending on the autopilot, it does not, it only tries to maintain altitude or heading at the expense of speed. When you trim down to maintain a speed you are setting a descent rate, which needs to be programmed into the autopilot to maintain otherwise the autopilot will adjust the controls to remain at the previously set altitude. Some autopilots have an auto-throttle system but not typically on GA aircraft.

Does being out of trim have an effect on drag?

@TomMcW I think that is a good topic for a new question!

Done

Why can't the autopilot move the trim wheel itself?

@ThrowawayAccount3Million Usually because the trim wheel is a separate cable system and would require an additional servo, adding cost/weight to the system. Multiply that by three if you have rudder, elevator, and aileron trims.

Is this the way airliner A/P's are designed, or does this only happen on GA aircraft?


text:
Is pilot alerted of any physical or structural failures?
Current crew alerting system messages, I believe, are displayed basing on the software detection of the failures basing on the sensor information.

What kind of structural failures can occur to an aircraft in flight? (I know one case an engine got separated and communicated to pilot as just 'engine fail' instead of 'engine broken or something that might indicate a physical disconnect'). The had me wondering what other structural failures are possible when in flight. When I say structural failure, I am talking about a physical disconnect, bent, etc. Is there a (remote) chance that a structural failure can be caused in an airplane.
If at all a structural failure occurs in the middle of flight, is there any mechanism to let pilot know about it.

Engine-off alerts are somewhat discussed here. There are indicators for the hydraulic system that tells if they have a problem or not.

"What kind of structural failures can occur to an aircraft in flight": Normally none. They happen only if some unexpected destructive event or fatigue occurs. The consequences of an unexpected cause are indeed nearly impossible to anticipate and difficult to sense. On the other hand whatever happens in an engine will likely create vibrations, so engine vibrations level is displayed to the crew with an alert threshold.

For #2.   The aircraft starting to tumble out of control might be a sign.  But there isn't much point to building in an alerting system when no possible pilot action can alter the outcome.

so if a part of wing is severed, I guess it is impossible to detect

First of all, the design is analyzed to ensure that no normal conditions will cause the structure to fail. In reality, any structure can fail, so any kind of structural failure is possible. Of course, we like our airplanes to not fall apart in flight, so many things are done to prevent that from happening.
When considering failure of structure, two main things are considered: what is the consequence of failure, and how easy is it to detect?
When the consequence of failure is high, of course the part is designed as much as possible to not fail. Parts are designed such that any damage will be contained and not become a widespread failure. Another approach is fail safe structure, such that if part of it fails, other parts will still carry the load until an inspection will find it.
Aircraft are inspected regularly, in detail ranging from a regular walkaround by the crew before/after a flight, all the way up to a D check where the whole airplane is basically taken apart, inspected, and put back together. These inspections are designed to find structural issues and fix them.
Some failures are obvious, such as large parts departing the airplane. As Carlo mentioned, there may be many clues if this happens. A failure of the fuselage pressure vessel will result in depressurization. Structural failure may cause other systems to be damaged, such as hydraulics, control surfaces, or electrical systems, which will trigger warnings in the cockpit. In these cases, the parts are designed to allow the plane to safely land, after which it is assumed the damage will be repaired. Of course, this doesn't always happen, but structure is designed to handle much more load than most flights ever experience.
Other failures may not be apparent at all. In these cases, special inspections may be required to check for issues, or the failure must not compromise the structure until a regular inspection will find the issue.

At the risk of sounding like a smartass, the alert is generally in the form of a loud BANG! or CRACK! followed, at best by different handling characteristics and increased wind noise/vibration, at worse, departure from controlled flight.
It also depends on the size of the aircraft, and how far away from the damaged structure the flight crew is or what systems were affected as a result. I remember that Colonel Al White who was captain for the XB-70 bomber prototype during the June 8, 1966 mid-air collision mishap, stated that he felt a light thump in the cockpit at time of impact, but neither he nor Major Carl Cross in the right seat thought anything about it until other aircraft in the formation were calling out that there was a midair collision on the radio.  It was much more obvious when the large airplane departed controlled flight.

That reminded me of Aloha flight 243.

If you add that there's no electronic warning as it's usually in the form of a Big Bang I'd say your answer should be accepted!

I suppose you might get an electronic alert, if a structural failure, say, compromised hydraulic lines, or some other system, and that set off a master warning, or some other alert in the cockpit that something was wrong. But typically structural failure sounds like a bang or ripping and tearing sound as the airplane breaks apart.


text:
What causes a military aircraft to be decomissioned?
The Harrier jump jet, while still in use in other countries, has been decomissioned in the UK, as the article below from the DailyMail mentions.
The Harrier jump jet for sale on eBay at £69,999
What are the causes of a still capable aircraft to be decomissioned?
Is it due mainly to political reasons, like this other article suggests in the case of the Harriers?
US military to buy Britain's scrapped fleet of Harrier jets

The US military has agreed to buy Britain’s entire fleet of Harrier
  jump jets after they were controversially scrapped under the
  Government’s austerity measures.

In action: Harrier T2 XW269 on active service in Gutersloh, Germany

I think it's a good question, but really hard to answer in general. I guess for most military aircraft it's a mix of obsolescence, cost cutting and changing requirements but that's dependent on the case. (For example: the Harrier is seen as useful only in certain applications, and the B-2 saw only a few built because the situation had changed by the time it reached production.)

This may be too broad as-is; the decision process is entirely different for commercial planes owned by companies, military planes owned by governments, and private planes owned by individuals.

In the Harrier's specific case, it was due to budget constraints. The UK Government decided to cut costs by scrapping the UK's Aircraft Carriers at the time. The Harriers operated from the carriers, so there was no use for them. The budget was transferred toward building the new Queen Elizabeth class carriers, which will carry F-35's

Well, there's also inter-service rivalries at work. The RAF was instrumental in dismantling the Royal Navy's air capabilities. There are many reasons why governments scrap aircraft, it's not always logical or sensible to those outside the process.

I would agree with @JonStory, as is says in your quote, "...scrapped under the Government's austerity measures". I think you answered your own question.

@GdD - that makes little sense considering the RN will have significantly more air power in 2 years time than in any point in her history...

Not true, the RAF and the RN are expected to have a "joint force" on the carriers. the RAF is likely to actually own the airplanes.


text:
Can trimmer and throttle be used for controlling large aircraft
During my PPL training, We practiced some emergency procedure on Piper cherokee and simulated flight controls failure.
My instructor asked me to sit on one hand and place the other on the trimmer.
I had to fly the aircraft using only rudder and trimmer, using trimmer and power for pitching and rudder for direction until short final.
My question is... Can the same procedure be exercised on big aircraft like Boeing?

Look no further than here for an answer.

Studies by Burcham and Fullerton at Nasa Dryden: Controlling crippled aircraft with throttles and Manual Manipulation of Engine Throttles for Emergency Flight Control

To some extent, yes. There was United Airlines Flight 232 in which catastrophic failure in the tail engine destroyed all the hydraulic lines. The plane was crash-landed in Sioux City, Iowa after the pilots controlled the aircraft using only the two intact wing engines. The trimmer might not produce as large as an effect on larger aircraft as on smaller ones; control of the engines would be enough because they affect the aircraft more than the trimmer due to their thrust.
However, if the damage is so severe that a control surface is destroyed or locked into a certain position, the engines might not be able to counteract that.

The trimmer must produce larger effect on a jet than it does on a small plane, because the range of operating speeds is larger. Jets usually trim by moving the whole horizontal stabilizer rather than trim tabs to get sufficient trim authority and less drag.

@JanHudec The effect is not as much as the effect by the engines.

The effect is different than the effect of the engines. The main problem with it is that on most aircraft the trim is actuated by the same hydraulic circuits as the elevator, so if you have a total hydraulic failure, you've lost both elevator and horizontal stabilizer trim.


text:
How do I determine the center of gravity on aircraft?
What are the parameters considered and what formulas are used to find the Center of Gravity on an aircraft?

Typically you take where the manufacturer tells you the CG is, and add the CG's of the stuff you are adding in, like fuel, people, and baggage. The POH has formulas for this specific to each aircraft. Are you asking how the manufacturer finds the CG?

You ask particularly about the Center of Gravity (CG), but not the weight. Any CG calculation will necessarily include the weight as a component of the calculation; therefore I refer to the calculations as weight and balance (or CG) calculations.
The two primary ways that weight and balance measurements are determined are by physically weighing the aircraft and by mathematically calculating based on additions or subtractions to the previous weight. The former is performed by maintenance personnel and forms the empty weight and CG for any given aircraft. The later may be performed by either flight operators or by maintenance and uses the predetermined empty weight and CG as the basis for the calculations; this may be needed due to the installation of new equipment, for example, or a pilot may do such a calculation to determine the weight and CG for a given flight with, for example, fuel and passengers.
For maintenance operations, most aircraft have a section in the maintenance manual or operating handbook that prescribes the process. For flight operations, most aircraft have an information manual or operating handbook that will prescribe the calculation.
One necessary parameter is an established reference datum.
The basic formula is:
$$
C_G=\frac{(W_1\cdot A_1)+(W_2\cdot A_2)}{W_1+W_2}
$$
Where:

$W_n$ is the weight of the aircraft scale's measurement, predetermined empty weight, fuel, or payload, etc.
$A_n$ is the moment arm, in reference to the datum, of the of the aircraft scale's measurement, predetermined empty CG, fuel, or payload, etc.
$C_G$ is the solution to the center of gravity for the aircraft.

The cancelled FAA Advisory Circular AC65-9A contains an entire chapter (Chapter 3) dedicated to the subject of weight and balance, including exactly what you ask, the parameters that must be considered and the formulae used to perform the calculations. Though cancelled, the AC is still available online (I have a print copy).

EDIT: See the comment below from @voretaq7 referencing the excellent "Aircraft Weight & Balance Handbook" quoted here:

In addition to AC 65-9A the current Aircraft Weight & Balance Handbook
  (FAA-H-8083-1A - available here) contains more than you ever wanted to
  know about computing weight & balance.

This is currently the direct link to the PDF of that document.

In addition to AC 65-9A the current Aircraft Weight & Balance Handbook (FAA-H-8083-1A - available here) contains more than you ever wanted to know about computing weight & balance.

It is not fully clear to me whether you are interested in determining the CG (Center of Gravity) of a new design or an existing one, I guess is the latter.
In this case the main parameters are:

The Payload (i.e.: people, cargo, other (e.g.: ammunitions))
The Fuel

The manufacturer provides you with tables to determine the CG due to the different loading condition (combination  of the aircraft payload and fuel).

For light planes, you can get a good approximation with a bathroom scale. First you put it under the front/tailwheel, note the force F1, and then do the same for one of the main wheels. You note the force and take double that value, F2.
The CG is in a point of a line along the fuselage, where the moments of F1 and F2 have the same value. It's elementary algebra for 10-year olds...

Very simple answer, but one familiar with modelers, is to lift it off the ground.  Keep in mind your horizontal (front to back) CG is normally slightly forward of your Clift, and is "balanced" by the angle of incidence, or trim of elevator.
There is also a vertical CG (top to bottom) which
should be compared with your center of frontal drag.
Note most aircraft tail designs are assymetric, with
more "up" than down, also adding to the up trim.
The drag from the vertical stabilizer is therefor
more efficiently used, saving some up trim on the horizontal.
For full scale, there are formulas in the POH based
on the CG before loading fuel, people, luggage, etc.
Very important pre-flight task to check this.


text:
How much water is produced in jet exhaust?
If a jet has typical combustion efficiency, what equivilent amount of liquid water will be created per gallon of jet or kerosene fuel. An acceptable answer can be a ratio in weight or volume to the original unburned fuel. For example an answer might be 1 part in 50 by volume or 1/4oz per pound is converted to water.
An answer in "water vapor" volume should be avoided because gas volume changes dramatically with altitude (pressure) and temperature.
If you are also able to show a comparison for gasoline, that would be even better.

Although I'm not sure about the background of your question, since you posted a picture of contrails: note that only a little portion of the water of the contrail is water resulting from the combustion of fuel. Most of its water was already present in the atmosphere before and the exhaust products (water, soot) were just acting as a seed for forming dropplets or ice crystals. I.e. choosing a different fuel has problably very little effect concerning formation of more or less contrails.

I mean the amount of water produced by the combustion of fuel has probaly not much effect. Much more important is the production of less soot: see this joint study of DLR and NASA about formation of contrails.

Note that the water will typically be heavier than the burned fuel; remember, it burns in the presence of atmospheric oxygen and you've got to take that mass into account as well.

Typical fuels consist mainly of carbon (C) and hydrogen (H). The amount of water that will be produced from combustion is dependent on the ratio of carbon to hydrogen. Taking a general fuel with a hydrogen to carbon atom ratio (H/C ratio) of $r$, the combustion looks like
$$ CH_r + \left(1+\frac{r}{4}\right)O_2 \rightarrow CO_2+\frac{r}{2}H_2O$$
For gasoline fuels, the H/C ratio seems to be around 1.8; for kerosene fuels, around 1.9. Based on this data, kerosene fuels produce a bit more water than gasoline fuels, in molar quantities.
We can convert this to weight by using the molar weights of carbon, hydrogen and oxygen, which are approximately 12, 1 and 16 gram per mol respectively. We can then find out that we will have
$$ m_\textrm{fuel} = 12+r \\
m_\textrm{water}=\frac{r}{2}\cdot(2+16)
$$
or, a water-to-fuel mass ratio of
$$\frac{m_\textrm{water}}{m_\textrm{fuel}} = \frac{9\cdot r}{r+12}$$
For gasoline, this would be about 1.17 kg water per kg of fuel. For kerosene, it is about 1.23 kg water per kg of fuel.
Side note: I found very different H/C ratios for gasolines, ranging anywhere from 1.3 to 2.1. I do not know if this based on actual variations (e.g., benzene has a H/C of 1, but hexane 2.33, despite having the same carbon chain length of 6) or if my quick literature search yielded bogus results. The value for kerosenes (and related diesel fuels) seems to be pretty steady around or slightly above 1.9.

+1 for citing papers. At to composition: Jet fuel is specified by its properties, not by its molecular composition. Depending on where you are in the world, how it has been stored and what batch it came from, it will be different. Chevron published a Technical Review on jet fuel around 2005/2006 which gives a nice overview of the properties used in the assessment of jet fuel.

"kerosene" (at least in English) describes more of a particular substance. "gasoline", on the other hand, describes a purpose for which a number of substances can be blended. Hence the differences are to be expected. On the other hand, both specs are rather loose so solid numbers seem suspicious anyway : )

@Agent_L, note however that jet and diesel fuels are much wider cuts—i.e. containing wider range of constituents—than gasoline, because these engines are much less picky about what they burn.

According to Wikipedia Kerosene consists of molecules containing 10..16 C-atoms.
Assuming that the molecules are mainly Alkanes with formula $C_nH_{2n+2}$:
$C_{10}H_{22}$: 10 * 12g + 22g = 142g Alkane contains 22g H which burns to 22g + 11 * 16g = 198g $H_2O$
Ratio Water/Kerosene = 198/142 = ca. 1.39
$C_{16}H_{34}$: 16 * 12g + 34g = 226g Alkane contains 34g H which burns to 34g + 17 * 16g = 306g $H_2O$
Ratio Water/Kerosene = 306/226 = ca. 1.35
I.e. you get something between 1.35g and 1.39g Water per 1g Kerosene.
Comparison to Gasoline:
Main difference of Gasoline is that molecule chains are shorter (4-12 C-atoms).
I.e. H/C-ratio will be higher, i.e. Gasoline will yield more water per fuel than Kerosene.

There should be around 10-25% (max) of aromatics in conventional jet fuel to ensure seal swell. At the moment aromatics are added to synthetic fuels - but this may change in the future. A paper on some fuel properties/assessment: https://pubs.acs.org/doi/abs/10.1021/ef101520v

Aromats (having smaller H/C-Ratio; ca. 1) would shift the resulting Water/Fuel ratio to smaller values.

There are slight variables making the calculation a little inexact, but this illustration shows a 1:1.24 ratio.

Glad to see my calculation is supported by this graphic within 1% accuracy. You should really indicate the source where you found it. A small note is that the water production is largely independent on the operating conditions, as the only other reaction product that has hydrogen in it would be the CxHy contribution which is less than 1% of the combustion products.

@Sanchises actually less than 0.1% (10 kg would be 1% of the initial 1 tonne, and even less of the 4.5 tonnes of reaction product).

@MartinBonner Yeah that should have been ‰.

This may be a silly question, but if a modern engine produces 1.24t of water per 1t of kerosene consumed, why aren't airports much foggier? Does the water just not condense fast enough at ground level?

@Jules most of the fuel is burned at high altitude. Very little fuel is used to taxi and idle. Takeoff fuel burn rate would be higher than cruise, but not terribly much. A quick search tells me a 747 burns 1 gallon per second in round numbers, so consider takeoff might be as much as double (probably not) and you're looking at an order of magnitude estimate around 20lbs of water per second. Then consider how quickly the plane is then relatively far away from the airstrip. Thinking about the volume / mass / energy of the atmosphere is often hard to put into perspective how large it really is.

@Aaron well-explained, thanks!

Your answer could be much better if it would include a source for the picture

Swiss Air uses a similar version of the image: https://www.swiss.com/corporate/en/company/responsibility/ecological-responsibility... with some of the smaller values reversed.


text:
Why are turbofan-engined Caravelles' engine pylons open at the back?
The engine pylons of turbofan-engined Caravelles are open at the very back, looking for all the world like someone'd simply taken a saw and sliced off the rearmost portion of the pylon:

(Image originally by clipperarctic at Flickr, via Russavia at Wikimedia Commons; cropped and annotated by me.)

(Image originally by André Cros, via Donna Nobot at Wikimedia Commons; cropped and annotated by me.)
In contrast, Avon-engined Caravelles (the Caravelle I-VI) have a non-apparently-sliced-open style of engine pylon which tapers to a closed trailing edge in the normal manner:

(Image by Mattias Björklund (Sgt. Oddball) at Wikimedia Commons.)
Given that it seems intuitively as though the turbofan-engined Caravelles' chopped-open engine-pylon trailing edges would significantly increase drag (which is generally considered a bad thing unless you can make it so that it only increases drag when you want increased drag), why are these Caravelles' engine pylons built like this?

Openings like that area almost always an exhaust for a heat exchanger.


text:
Do any aircraft windshields have anti-reflective coatings?
Anti-reflective coatings are used in many applications to reduce glare and prevent double images.  Double images in particular are a concern because of the several layers involved in a cockpit window.  More high-tech optical coatings can also have other desirable properties such as reducing UV and IR light transmission. There is some evidence of them being used in industry, like this patent for a windscreen. On a related note, cockpit instruments have a limit for maximum reflectivity of their glass/plastic surface and sometimes use anti-reflective coatings.
I started thinking about this because of a recent question on why windscreens aren't more reflective.
However, I can't find many examples of anti-reflective coatings (possibly because of the harsh environmental conditions?).  UV and IR protection seem to be mostly through tinting instead of coating.
Do any production aircraft use anti-reflective or other coatings?  Why or why not?

It looks like one of the windows on this 787 is reflective coated

Some military aircraft have a gold coatings from what I know.

@RonBeyer Yes, good find.  The 787 seem to have a gold-based heating film that also serves to block infared light: http://www.aerospacemanufacturinganddesign.com/article/aerospace-manufacturing-design-boeing-ppg-787-amd/

@SMSvonderTann I'm familiar only with the F-16 "Have Glass" golden coating designed to reflect radar.  If you have any good sources, go ahead and put them as an answer.

Related questions for fighter jet canopy materials, tinted windows.

I am not sure but almost anything reflects light (even dust in air reflect light), even just a little bit. So a coating always reflects some light away which making it not so possible to make something anti-reflective. The work should be done on the glass itself to make it less reflective overall.

@Hezzte You seem to misunderstand how anti-reflective coatings work. I'd suggest you read up some on them before speculating.  Anti-reflective coatings have been around for over a century and are used on many glasses, and are even listed in the patent I linked in my question.  This is not novel or extraordinary technology by any means.

An antireflective coating for the F/A-18's canopy was successfully tested sometime in the 1990s; no word on whether it made it into production, though.
Wikipedia claims that "[n]early all glass windows in large aircraft have an anti-reflective coating", but provides no source for this assertion.


text:
Why shouldn’t the A330/40’s speedbrakes be used in alternate law?
On page 30 of the ATSB report into the QF72 accident (page 50 of the PDF file of the report), in a table describing the actions required of the flightcrew in response to some of the ECAM messages displayed during the flight, there is the following entry:

ECAM message
[...]
Required actions

[...]
[...]
[...]

F/CTL ALTN LAW (PROT LOST)
[...]
Do not use speed brake [...] Maximum speed limited to 330 kts / Mach 0.82

[ATSB Transport Safety Report AO-2008-070, page 30 (PDF page 50), Table 6.  Table headings bolded in original; all other bolding is my own.]
Why aren’t the A330/40’s speedbrakes (the term used for the aircraft’s flight spoilers, when deployed symmetrically to increase the aircraft’s drag and reduce its lift) supposed to be used when in alternate law?


text:
Would a rotating skin design mitigate the heating problem limiting hypersonic flight?
Would a rotating skin design - where the skin of the aircraft rotates (powered either by an electric motor or the colluding wind) - mitigate the heating problem limiting hypersonic flight?
Would a liquid cooling system (to cool the skin as it rotates along the aircraft) add additional benefit?

I think I see where you're heading. By moving the skin, you're taking a piece of skin away from the highest heat spot and allowing it to cool before eventually moving it back into the blast furnace of the leading edges. However, I would imagine that the mechanisms to make the skin crawl :) would be far more complex than most of the rest of the aircraft. Additionally, modern aircraft design uses the skin itself as a stressed member - detaching it from the underlying structure would, essentially, cause the structure to fail.

A liquid cooling system worked for the SR-71 without rotating the skin. Fuel was piped through capillaries in the skin for the dual purpose of cooling the skin and heating the fuel

related, if not duplicate: https://aviation.stackexchange.com/q/44600/1467

TL;DR Probably not.
Assuming we would have a very good mechanical solution/design we would still need a way to get rid of the heat, i.e. a heat-sink.
Imagine the panel (or what ever we will call a part of the skin) is rotated away from the flow (into the inside of the vehicle), than we would need to cool it but where would we store the heat? We cannot extend a heat-exchanger. So the inside of the vehicle would heat up indefinitely.
Essentially we need to come up with a way to discharge entropy from the vehicle. This can be done by either a heat- or mass-flux from the vehicle into the ambiance.
One possibility is related to your second idea: using a cooling system. This requires you to have a fluid propellant. Using the fluid propellant as a cooling fluid first you would not only cool your airframe but you would also enhance your efficiency. This also solves the heat/entropy problem because you would discharge mass (the propellant).
But given the fact that we assumed to have a matured mechanical design of the ‚skin-rotation‘ mechanism you could have multiple panels with ablative coating. So after the first panel is consumed you could (hypothetically) rotate a new one into the flow.


text:
What does Airbus means by long product cycles?
I was reading this article. The article was about issues that Airbus may face if the UK does a 'Hard Brexit'.  At the very end of the Article though, Airbus says

Airbus makes wings for all its jetliners in the UK. The France-based
  company campaigned against Britain leaving the EU ahead of the UK
  vote, but has said it would not make abrupt changes to its operations
  there due to long product cycles. - foxbusiness.com

Can anybody elaborate what the company means by long product cycles?

This question really isn't about aviation per se, and might be better asked over at ELL. They're very friendly and have many questions about English expressions like this.

It's part English and part how airplane manufacturing works. If I were to take ELL, while they may be able to fill in 'long product cycle' - probably meaning it takes a long time for things to come to fruition, how this industry works and whether that term is correct can only be accessed by somebody who's in the business.

A Product Lifecycle is the process through which a consumable product goes through from conception to retirement. A "long product lifecycle" means that the manufacturer takes steps to keep that product (in this case, an airplane) in manufacturing for an extended time. The A-320 for example first flew in 1987 and is still being manufactured with no plans for retirement.

It doesn't have any special meaning in Aviation compared to any other industry. As Ron Beyer says, it simply means the product will be in production for a long time before a replacement is sought.

A long product cycle means that the lifecycle of a product spans a long time. Aircraft models are often produced for more than 30 years (the 737 and 747 are now approximately 50 years in production).
At the same time, the capital expenses to build a production line are enormous. And in starting up a production line there are always teething troubles which take time and money to iron out. Therefore it would be a waste of capital to move a production line from the UK to mainland EU.
That only is viable when a new aircraft model is introduced and an old model is phased out. New models don't appear every year and it seems that most of the current models are still selling well so unless there will be a huge cost associated with keeping the production line in the UK (e.g. taxes, increasing labour cost, unfavourable exchange rate) no changes are to be expected soon.

Costs may actually decrease after UK scrub taxes on exported products to increase its attractiveness related to EU companies. The same for RR competing against Safran (CFM international). Airbus customers may be very happy.


text:
Why do aircraft sometimes bounce while landing?
I've seen a few youtube videos where an aircraft bounces once/twice while landing on the runway. Why does this happen?

To err is human.

I found it quite interesting and right-on-subject: https://www.boldmethod.com/blog/2014/01/the-danger-of-porpoise-landings/

The FAA covers this in the airplane flying manual quite nicely:

Bouncing During Touchdown
When the airplane contacts the ground with a sharp impact as the result of an improper attitude or an excessive rate of sink, it tends to bounce back into the air. Though the airplane’s tires and shock struts provide some springing action, the airplane does not bounce like a rubber ball. Instead, it rebounds into the air because the wing’s AOA was abruptly increased, producing a sudden addition of lift...

There is some more info in the doc worth reading as well.
In the simplest terms this can be one of the outcomes of an unstable approach; oftentimes, when the approach speed is too high, bouncing will result. Touching down on the nosewheel first can also cause a very dangerous bounce case known as a porpoise.
In some cases wind shear can cause bouncing. In this case the shear may cause an unexpected downdraft, forcing the plane down onto the runway vertically and inducing a bounce, or it may cause an updraft leading to an overcorrection by the pilot to descend; when the updraft stops, the plane falls rapidly.

By the way, a bouncing landing is referred to as "N landings, one arrival" where N = the number of bounces.

This one is quite well known https://www.youtube.com/watch?v=lhUO4pdGCf8 and nicely shows the amplification and landing on the front wheel. Some aircraft would not survive that.

@VladimirF I'm not sure that one did survive.  I mean, it landed, but I'll bet it took a good bit of work before that thing could fly again...

@DarrelHoffman the locals claim in the comments that this thing still (in 2019 - since 2006) is in that airport with the front chassis removed and unrepaired since that incident, and used as a learning illustration for other student pilots.

Seems to me it happens more often when there is a strong crosswind, I'm guessing because of a higher approach speed used.

Tail draggers are especially prone to bouncing when landed badly, if the main wheels touch down first.
Just before touchdown, the wing is at the right angle of attack to generate enough lift to support the weight of the plane. If the main wheels touch down first, the tailwheel descends a bit further, which increase the angle of attack of the wing. Lift is then greater than weight, so the plane takes off again - albeit somewhat temporarily.
It's similar to landing tricycle undercarriage on the nose wheel first, except a much easier mistake to make.

authors: Vladimir F Героям слава


text:
Is it legal to turn off ADS-B before landing and only after takeoff in Europe (GA)?
I noticed in https://globe.adsbexchange.com that all aircraft of a certain company do not show ADS-B signal on almost all take offs and landings (around 3000-4000 feet) over the past 6 months.
Assuming that it is company policy and not a signal issue, would this be legal? i.e. what are the specific requirements regarding the point during the trip that the ADS-B is expected to be turned on.
If relevant, they are using mostly Cessna 650 Citation (C650), operating in DE,UK,DK,etc.

What company? Tail numbers?

That sounds pretty unlikely. More likely, the hobby website you are talking about does not have perfect coverage, so it will lose tracking of certain aircraft below certain altitudes in certain areas.

@60levelchange, you are right, I phrased the question under a set of assumptions that are not necessarily true. I re-phrased the question accordingly.

I've seen this happen at Mumbai airport too and I can confirm it's related to certain plane models and their specific transponder characteristics. They aren't turning off ADS-B but instead, the ADS-B coverage for those planes is poor. I've noticed that when this happens (on FR24 at least) you can track the planes using MLAT.
I'm quite sure it's not the pilots actually doing anything with the transponders, the altitudes you've mentioned are also pretty similar to what I've noticed a rare few times at Mumbai airport.
I had posted this exact same question a few months ago but it got downvoted and I had to remove it.

Hmm, it doesn't make much sense to me that you'd be able to track it with MLAT if it was still broadcasting ADSB - you only need one receiver to detect ADSB because all the data you need is in the packet whereas you need several for MLAT

@Dan, didn't get you.

This answer does not make sense. If you can track an aircraft with MLAT, it means you are receiving messages from the transponder at multiple receivers simultaneously. This means that the coverage must be good. If the some of those message would be ADS-B messages, they would be received as well. So you can't have poor ADS-B coverage and do MLAT at the same time. If you can do MLAT, you can also receive ADS-B messages, unless the aircraft isn't broadcasting any.

@DeltaLima I do not know, it happens with certain aircraft but most aircraft are not affected by this poor ADS-B coverage


text:
What is the relationship between "automatic flight control system AFCS", "autopilot", "flight director" and "flight management system FMS"?
I'm studying aircraft maintenance. I looked in my books and searched the web but couldn't make a big picture of the concept of autopilot.

Specifically what is its relation to modern systems like FMS.
Is the AFCS an independent system or does it take orders from the FMS?
What are AFCS subsystems?
Is the flight director the old version of FMS?

Since you are studying maintenance, I would suggest you obtain a copy of ATA iSpec 2200, Information Standards for Aviation Maintenance ATA Standard Numbering System.  Most large aircraft manufacturers and airlines use the ATA Standard Numbering System in organizing their maintenance documentation.
To your specific question, the ATA spec lists the following:
The FMS is the primary tool for managing the navigation.  It is part of the larger Navigation system which includes the aircraft attitude, air data, and positioning sensors. The FMS supports flight planning (path definition) and integrated position determination.

Chapter 34 NAVIGATION, section 60 Flight Management Computing.
That portion of the system which combines navigational data to compute
  or manage the aircraft's geographical position or theoretical flight
  path. Includes items such as course computers, flight management
  computers, performance data computers, and associated control display
  units, warning annunciators, etc.

The FM function outputs a desired path and guidance data to the displays and AFCS. The FMS is one source of path and target (altitude and or speed) data for the AFCS. The other input source is by crew input via the Mode Control Panel (MCP).
The AFCS is functionally independent from the FMS and primary flight control system (FCS), though it is sometimes physically integrated with one or the other or both.
The AFCS has two major components, autopilot and auto throttle. Autopilot includes the Flight Director.

Chapter 22 AUTO FLIGHT, section 10 Autopilot
That portion of the system that uses radio/radar signals, directional
  and vertical references, air data pitotstatic, computed flight path
  data, or manually induced inputs to the system to automatically
  control the flight path of the aircraft through adjustment to the
  pitch/roll/yaw axis or wing lift characteristics and provide visual
  cues for flight path guidance, i.e.: Integrated Flight Director. This
  includes power source devices, interlocking devices and amplifying,
  computing, integrating, controlling, actuating, indicating and warning
  devices such as computers, servos, control panels, indicators, warning
  lights, etc.

and

Chapter 22 AUTO FLIGHT, section 30 Auto Throttle
That portion of the system that automatically controls the position of
  the throttles to properly manage engine power during all phases of
  flight/attitude. This includes engaging, sensing, computing,
  amplifying, controlling, actuating and warning devices such as
  amplifiers, computers, servos, limit switches, clutches, gear boxes,
  warning lights, etc.

The AFSC computes the necessary control inputs to fly the aircraft along the defined path and meet targets.  It provides control inputs into the flight controls (ATA Chapter 27) or in the case of the Flight Director, it generates a display that directs the pilot to make the needed control inputs.
This steering will steer the aircraft to the path defined by the FMC, or it can steer to a simple path entered by the pilot directly into the AFCS via the MCP.  In older aircraft, the autopilot steers by using a series of special actuators to manipulate the controls. In modern fly-by-wire aircraft, it provides input directly to the FCS as the FCS already has the necessary actuators.
Auto Throttle provides throttle adjustments to meet speed targets from the FMC or MCP.
By the ATA standard, the FMS, AFCS, and FCS are three distinct systems. As noted above, modern fly-by-wire systems are beginning to blur the boundaries.  Without the need for separate actuators for the AFCS, it just becomes software much of which is getting merged with the primary FCS.

Not all AFCS include an auto throttle or auto thrust system. Not all autopilots include a flight director.

Thank you so much Gerry . it was very helpful . and is the ATA iSpec 2200 available to public for free or it's a published book ? I googled it and couldn't find anything . thank you

It's a publication of the Airlines for America (www.airlines.org) which was formerly known as the Air Transport Association (ATA). Most of their industry information can be purchased through their e-business portal (www.ataebiz.org). The documents cover a wide range of airline operations. Unfortunately, they're not cheap. I have the iSpec Extract, which is just the structure and definitions without all the details. You might see if your school library can obtain a copy. Alternately, you can just look at a Boeing or Airbus maintenance manual if you have access to one.

The FMS is the whole system that includes everything from the knobs the pilots touch, the CDU's with keypads and screens, the black-box computers, everything.
A flight director is, conceptually, an autopilot without the actuators -- it presents guidance to the pilot, who follows that guidance to match what an autopilot would be doing.
The AFCS may take orders from the Flight Management Computer(s) (which are parts of the overall FMS), or it may be set to fly simply a given heading & altitude -- whatever mode the pilot has selected.


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What is the second "door" on the DC-3?

Is under the cockpit used to access avionics on the ground or something like that ?

I feel like we have seen one of these before...  or was that for the DC4?

In 1974 I was hauling checks in the Northeast and we routinely used this door to board the plane. When it was frozen shut or had a latch problem, which happened, then we crawled around the sacks and cargo netting. If everything was working we used the front door.  Schedules were tight and we were normally getting our clearances while the bags were getting loaded and offloaded.

It seems to be an access door for the pilots, judging from this photo. Not a super easy entrance, the door in the back is a bit lower down.

As the caption indicates, it allowed access to the flight deck when the cabin was packed with cargo. My dad, an A&P who spent 20+ years working on DC-3s, referred to it as the "meat grinder door" -- not to be used with the left engine running.